Method of anodically etching aluminum foils at elevated temperatures in an electrolyte including chloride and sulfate ions



United States Patent 3,321,389 METHOD OF ANODICALLY ETCHING ALUMI- NUM FOlLS AT ELEVATED TEMPERATURES IN AN ELECTRGLYTE INCLUDENG CHLO- RIDE AND SULFATE IONS Daniel J. Anderson, Indianapolis, Ind., assignor to P. R. Mallory & Co., Inc., Indianapolis, Ind., a corporation of Delaware No Drawing. Filed July 20, 1964, Ser. No. 383,960 3 Claims. (Cl. 204-141) The present invention relates to improvements in the etching of aluminum and aluminum-base alloys and more particularly relates to the treatment of aluminum and aluminum-base alloys to increase the effective surface area thereof.

Aluminum, and in particular aluminum foil so treated is of particular value in the construction of electrolytic capacitors which include an aluminum electrode having an electrically formed dielectric oxide film thereon. The capacitance of such a capacitor depends upon the area of filmed electrode surface in contact with the electrolyte between it and the complementary electrode.

It is known in the art to increase the efiective surface area of aluminum, and in particular aluminum foil, by both a chemical etching process and by an anodic electrolytic etching process in which a smooth direct current is employed or in which unidirectional uninterrupted current of fluctuating magnitude is employed. The increase in the effective surface area of aluminum so treated is largely due to the production of very fine and relatively deep pores. However, it has been found that the addition of sulfate to the etching solution produces an increase in capacitance and is particularly applicable for increasing the effective surface area of aluminum processed for low voltage formation.

Therefore, it is an object of the present invention to provide an improved process for etching aluminum to increase the effective surface area thereof.

It is an object of the present invention to provide an improved process for etching aluminum to increase the average capacity of etched aluminum foil processed especially for low formation voltages of 100 volts and less.

It is an object of the present invention to provide an improved process for etching aluminum which reduces capacity variations of a seasonal nature which occur in low voltage foil, due to variations in the temperature at which the etching brine is made up.

The present invention, in another of its aspects, relates to novel features of the instrumentalities of the present invention described therein for teaching the principal object of the present invention and to the novel principles employed in the instrumentalities Whether or not these features and principles may be used in the said object or in the said field.

With the aforementioned objects enumerated, other objects will be apparent to those possessing ordinary skill in the art. Other objects will appear in the following description and the appended claims.

Generally speaking the present invention provides a method of etching aluminum foil to be formed at low voltages to obtain increased capacitance so that capacitor sizes may be reduced. The invention consists of the addition of small amounts of a soluble sulfate to the etching bath which is composed of an aqueous sodium chloride solution containing 16-24% sodium chloride by weight. Both sodium and potassium sulfates have been used, however, any soluble sulfate is suitable. The sulfate is mixed with the etching solution prior to the etching, and additional sulfate is added thereafter if necessary. The addition of the sulfate ion increases the electrical capacitance of the etched aluminum foil and particularly increases the 3,321,389 Patented May 23, 1967 capacitance of etched aluminum foil processed especially for low formation voltages.

The following table shows typical results obtained utilizing the aforementioned invention. Figures are given in microfarads per square inch at 30 volts formation.

Foil Purity Gms. NaSOr per 100 gms. N 2:01

99.93% Al 99.99% Al The addition of sulfate serves to increase the penetration powers of the etching solution thereby increasing the eifective surface area of the etched foil which, in turn, increases the capacitance. The etching process is carried out between 100 C. Current densities of 1 ampere per square inch and up and a cycle of 4-15 ampere minutes per square inch are utilized. The specific current density and time cycle depends upon the size of the foil. A unidirectional current is used throughout the operation.

It will be understood that the present invention as hereinbefore described is merely illustrative and not exhaustive in scope. Since many Widely different embodiments of the invention may be made Without departing from the scope thereof, it is intended that all matter contained in the above description shall be interposed as illustrative and not in a limiting sense. Having thus described my invention,

I claim:

1. The method of treating the surfaces of substantially oxide free aluminum foil having a purity of at least 99.93 percent used in a capacitor electro-formed at up to volts to increase the effective surface thereof by etching thereon a fine etch pattern comprising the steps of immersing a substantially oxide free aluminum foil having a purity of at least 99.93 percent in an aqueous bath containing about 16 to 24 percent by weight of an alkali metal chloride and about 0.1 to 10 percent by weight of a soluble alkali metal sulfate, maintaining said aqueous etching bath at a temperature of about 70 to 100 C., conducting between said substantially oxide free aluminum foil which is made anodic and a cooperating cathode a unidirectional current having a density of about one ampere per square inch of said metal surface and having a time cycle duration of about 4 to 15 ampere minutes per square inch of said metal surface thereby causing etching of said metal surface of said aluminum foil by a combined chemical and electrolytic action, continuing said etching of said metal surfaces until the capacity value of said metal surfaces is effectively increased by the formation of a fine etch pattern therein, and cleansing said aluminum foil of said aqueous etch bath.

2. The method of treating the surfaces of substantially oxide free aluminum foil having a purity of at least 99.93 percent used in a capacitor electro-formed at up to 100 volts to increase the effective surface thereof by etching thereon a fine etch pattern comprising the steps or immersing a substantially oxide free aluminum foil having a purity of at least 99.93 percent in an aqueous etching bath containing an alkali metal chloride and a soluble alkali metal sulfate, maintaining said aqueous etching bath at a temperature of about 70 to 100 C., conducting between said substantially oxide free aluminum foil which is made anodic and a cooperating cathode a unidirectional current having a density of about one ampere per square inch of said metal surface and having a time cycle duration of about 4 to 15 ampere minutes per square inch of said metal surface thereby causing etching of said metal surfaces of said aluminum foil by a combined chemical and electrolytic action, continuing said etching of said metal surfaces until the capacity value of said metal surfaces is effectively increased by the formation of a fine etch pattern therein, and cleansing said aluminum foil of said aqueous etch bath.

3. The method of treating the surfaces of substantially oxide free aluminum foil having a purity of at least 99.93 percent used in a capacitor electro-formed between about 30 and 100 volts to increase the effective surface thereof by etching thereon a fine etch pattern comprising the steps of immersing a substantially oxide free aluminum foil having a purity of at least 99.93 percent in an aqueous etching bath containing about 16 to 24 percent by weight of an alkali metal chloride and about 0.1 to 10 percent by weight of a soluble alkali metal sulfate, maintaining said aqueous etching bath at a temperature of about 70 to 100C, conducting between said substantially oxide free aluminum foil which is made anodic and cooperating cathode a unidirectional current having a density of about one ampere per square inch or higher of said metal surface and having a time cycle duration of about 4 to 15 ampere minutes per square inch of said metal surface thereby causing etching of said metal surfaces of said aluminum foil by a combined chemical and electrolytic action, continuing said etching of said metal surfaces until the capacity value of said metal surfaces is effectively increased by the formation of a fine etch pattern therein, and cleansing said aluminum foil of said aqueous etch bath.

References Cited by the Examiner UNITED STATES PATENTS 2,336,846 12/1943 Clark 204141 2,598,043 5/1952 Eichner 204-141 3,085,950 4/1963 Thomas et al. 204-141 FOREIGN PATENTS 467,024 9/ 1937 Great Britain.

JOHN H. MACK, Primary Examiner.

R. K. MIHALEK, Assistant Examiner. 

1. THE METHOD OF TREATING THE SURFACES OF SUBSTANTIALLY OXIDE FREE ALUMINUM FOIL HAVING A PURITY OF AT LEAST 99.93 PERCENT USED IN A CAPACITOR ELECTRO-FORMED AT UP TO 100 VOLTS TO INCREASE THE EFFECTIVE SURFACE THEREOF BY ETCHING THEREON A FINE ETCH PATTERN COMPRISING THE STEPS OF IMMERSING A SUBSTANTIALLY OXIDE FREE ALUMINUM FOIL HAVING A PURITY OF AT LEAST 99.93 PERCENT IN AN AQUEOUS BATH CONTAINING ABOUT 16 TO 24 PERCENT BY WEIGHT OF AN ALKALI METAL CHLORIDE AND ABOUT 0.1 TO 10 PERCENT BY WEIGHT OF A SOLUBLE ALKALI METAL SULFATE, MAINTAINING SAID AQUEOUS ETCHING BATH AT A TEMPERATURE OF ABOUT 70 TO 100%C., CONDUCTING BETWEEN SAID SUBSTANTIALLY OXIDE FREE ALUMINUM FOIL WHICH IS MADE ANODIC AND A COOPERATING CATHODE A UNIDIRECTIONAL CURRENT HAVING A DENSITY OF ABOUT ONE AMPERE PER SQUARE INCH OF SAID METAL SURFACE AND HAVING A TIME CYCLE DURATION OF ABOUT 4 TO 15 AMPERE MINUTES PER SQUARE INCH OF SAID METAL SURFACE THEREBY CAUSING ETCHING OF SAID METAL SURFACE OF SAID ALUMINUM FOIL BY A COMBINED CHEMICAL AND ELECTROLYTIC ACTION, CONTINUING SAID ETCHING OF SAID METAL SURFACES UNTIL THE CAPACITY VALUE OF SAID METAL SURFACES IS EFFECTIVELY INCREASED BY THE FORMATION OF A FINE ETCH PATERN THEREIN, AND CLEANSING SAID ALUMINUM FOIL OF SAID AQUEOUS ETCH BATH. 